The present invention relates to a through hole conduction structure of a flexible multilayer circuit board in which a high-density wiring is required and which has been worked out so as not to generate a crack in a through hole coating layer, and to a forming method thereof.
Generally, in order to manufacture a multilayer circuit board, an adhesive or an adhesive resin layer called a pre-preg is used on both sides of an internal layer material to laminate and mold a copper-clad laminate. In particular, when laminating a thin multilayer circuit board such as a flexible multilayer circuit board, cover films consisting of, e.g., polyimide resin as an interlayer insulating material are attached on both sides of the internal layer material, and an external layer material is laminated on the obtained product through the adhesive or the pre-preg.
In such a flexible multilayer circuit board, after performing lamination and molding as described above, a through hole process is carried out at a required position of the laminated circuit board and a through hole coating layer is formed in the through hole to attain conduction between the internal layer and the external layer.
However, since the cover film consisting of the polyimide resin has a high thermal expansion coefficient, it expands due to heat from the solder flow having a temperature of approximately 220xc2x0 C. at component mounting, and a crack is apt to be generated in the through hole coating layer. In the flexible multilayer circuit board in which a high-density wiring Is required, this can be a serious problem as a product.
It is, therefore, an object of the present invention to provide a through hole conduction structure of a flexible multilayer circuit board in which a high-density wiring is required and which is configured so as not to generate a crack in a through hole coating layer, and to a forming method thereof.
To achieve this aim, the present invention provides a through hole conduction structure of a flexible multilayer circuit board comprising an internal layer circuit board which can be a cable portion, and an external layer circuit board which can be a component mounting portion laminated on one side or both sides of the internal layer circuit board at a predetermined position, wherein, in the flexible circuit board having a through hole plated conduction portion formed at predetermined positions of the internal layer circuit board and the external layer circuit board, a surface protection layer formed on an external surface of a wiring pattern of the internal layer circuit board is formed in a region retreated toward the outside from a position of a through hole for the through hole plated conduction portion.
According to the above-described structure, it is possible to preferably eliminate a problem of generation of a crack in the through hole plated conduction portion in the prior art even if a polyimide resin cover film is used for the surface protection layer.
Further, as a forming method for that structure, it is possible to adopt a technique comprising the steps of: forming a required wiring pattern on one side or both sides of a flexible insulating base material; preparing an internal layer circuit board having a surface protection layer formed thereto on an external surface of the wiring pattern at a position retreated toward the outside from a position where a predetermined through hole is to be formed; laminating an external layer circuit board which can be a component mounting portion on one side or both sides of the internal layer circuit board in association with the position of the Internal layer circuit board where the through hole is formed; forming a through hole at predetermined positions of the internal layer circuit board and the external layer circuit board; and forming a through hole plated conduction portion on the inner surface of the through hole.
Since the surface protection layer formed to the internal layer circuit board is provided at a position retreated toward the outside from a position where a predetermined through hole is formed, it is possible to preferably eliminate the adverse influence of thermal expansion on the through hole plated conduction portion even if the polyimide resin cover film is used for the surface protection layer.